With the electrical parking brake, as with a conventional mechanical parking brake, there is the requirement for the mechanical braking system to be able to be maintained, for example the brake cable or also brake pads must be able to be replaced, renewed or adjusted. To this end it is necessary in a service mode to be able to place the parking brake with the aid of a release function into a completely opened position so that no forces act on the brake cable and the brake mechanism. By contrast with a conventional mechanical parking brake, with the electrical parking brake the completely opened position cannot simply be set mechanically. Instead it is necessary to put the electrical parking brake by electrical means with a corresponding release function into the completely opened position in the service mode. For safety reasons the parking brake may only assume this release position in exceptional circumstances if the motor vehicle is stationary and there is no danger of it being able to roll away for example. In this opened position a parking brake is opened so wide that any attached brake cable with which the brake shoes or brake pads of the parking brake are applied, can come loose from its brackets by itself, so that the function of the parking brake becomes ineffective.
It is further known that, to avoid the above-mentioned problems, with a known electrical parking brake the activation of the service mode is only enabled using a secure diagnostic command. The diagnostic command in this case is specified with the aid of an external diagnostic device which must be connected accordingly to the electrical parking brake. Such a diagnostic device generally functions only for a specific motor vehicle of an individual motor vehicle manufacturer and is unlikely to be available to all service workshops. Thus, for cost reasons, only a small proportion of the service workshops will be able to use such a proprietary diagnostic device.
Furthermore it is known that the electrical parking brake cannot simply be put into a completely opened position by mechanical means. This is because the emergency release of the brake mechanism subjects the actuator as well as the drive unit to very great mechanical stresses and can be damaged by this or a fault can be introduced into the braking system which leads to a registered incorrect setting. This incorrect setting cannot be corrected however without the specific diagnostic device. In this case too a corresponding diagnostic device is thus required to be able to reset the electrical parking brake by electrical means back into the functionally correct operating mode.